The present invention relates in generally to an electronically modulated prism, and more particularly, to a scalable and agile beam-steering device that uses an electronically modulated prism.
Precise and controllable delivery of light beams or other guided modes to a desired location has been an important topic with telecommunication, military and other general industrial applications. In the military application, the beam steering is a critical technique in an airborne, active, electro-optical (EO) system. Examples of missions requiring the beam steering technique include self-protection, such as infrared countermeasures (IRCM) and damage-and-degrade (D2) applications, targeting, intelligence, surveillance, and reconnaissance (ISR), infrared search and track (IRST), laser radar (LADAR), vibrometry, and free-space optical (FSO) communication.
Each of the above listed missions requires a unique beam-steering subsystem to adjust field of regard (FOR), slew rate or switching speed, and an upper bound on allowable jitter thereof, for example. The typical field of regard which challenges the coarse-steering element of a loop will be 0° to 45° in elevation and 360° in azimuth. The slew rate varies from 0.5 second for coarse-steering elements to less than 0.001 second for fine-steering elements. The jitter which challenges the fine-steering element of the loop typically ranges from several mrads for the more forgiving missions to tens of μrads for the more challenging missions.
There has been recent interest in development of novel beam-steering approaches in the industry. These approaches progress in novelty from non-gimbal designs (which involve conventional turrets without gimbal) to non-mechanical designs (which use no moving parts or at least no moving parts on a macro scale critical to tracking function), conformal designs (which may use gimbals or other mechanical elements conformal to the skin of the aircraft), and agile designs (which are conformal, non-mechanical and fully-digitally selectable). The conformal designs are advantageous to reduction in aerodynamic drag and in the radar cross-section of the aircraft. The agile approach would constitute an analogue to the available electronically-steered radar.
Practical solutions for agile steering have been developed to maintain full-steering, coarse-steering and requisite fine-steering field of regard. A variety of solutions have been proposed for agile elements dedicated to fine steering. For example, flexible waveguides and Bragg gratings have been applied to fine steering in telecommunications. Liquid crystal (LC) arrays, micro electro-mechanical (MEM) based and segmented arrays of optical fibers have been proposed to address the fine-steering problems. The baseline of the evaluation of each of the above fine steering techniques could be the conventional fast-steering mirror (FSM), which is able to steer over 1° to 3° with closed-loop bandwidths of about 500 Hz to about 2000 Hz for small-angle corrections.
The more mature agile techniques are the array-based elements that achieve fine steering by managing phase differences on a fine scale. As known in the art, the phase of a light beam traveling through an optical medium can be expressed by:φ(x)=(2π/λ)OPD(x)  (1),where φ is the phase, λ is the wavelength of the light beam, and OPD(x) is the optical-path difference along x-direction through the optical medium, which is either the lead or lag of a portion of the wavefront with respect to the rest of the light beam. Often time phase non-uniformity is undesirable, but can be used advantageously to steer a beam. A phase grating, for example, whether reflective or transmissive, and whether sinusoidal or blazed, can be used to deflect a portion of the incident wavefront in a chosen direction. Phased arrays of optical fibers, liquid-crystal arrays and MEMs based arrays can be used in this way for beam steering.
However, because these approaches lack the phase range, or “stroke” to modulate the whole wavefront sufficiently steer the incident light beam at a significant angle, the steering elements are subdivided into smaller arrays that operate on subaperture portions of the larger wavefront. The phases of these subaperture portions are locked or phased within a modulo of 2π shift. The subaperture division leads to undesired effects. For example, a wavefront that is subdivided will experience diffraction losses versus the original wavefront; and additionally, each subaperture will have a larger, far-field divergence compared to that of the original wavefront. One of the most vexing issues associated with the subaperture effect is the true time delay (TTD) experienced progressively across the wavefront. In the free-space optical communication missions or in any mission concerned with very fast ( about tens of Gb per second) modulation of waveforms, phasing of subapertures will cause some parts of the wavefront to reach the receiver detector at a later time than other parts of the wavefront, leading to signal “smearing” in time.
It is therefore a substantial need to develop an agile-steering element, at least for fine steering, which does not rely on the division of the steered wavefront into subapertures. The element will steer in the same fashion that a glass prism do for steering a beam. The agile-steering element can function as a dynamic prism.